1. Field of the Invention
The present invention relates to an image processing apparatus, a control method of an image processing apparatus, an image forming apparatus, and a storage medium.
2. Description of the Related Art
In recent years, image quality enhancement of an output image and speeding-up of image formation are required more than ever for image forming apparatuses such as printers and copying machines which adopt an electrophotography system, inkjet system, and the like. Especially, in case of a multi-color image forming apparatus of the electrophotography system, a technique using a plurality of photosensitive members corresponding to different colors so as to attain speeding-up is known. Such image forming apparatus corresponds to a tandem type which attains multi-color printing by forming toner images of respective colors on respective photosensitive members, and transferring these toner images in turn from the photosensitive member onto a transfer member or a printing material so as to be superposed on each other.
However, an image forming apparatus often suffers a tilt and curvature of a scanning line due to various causes generated by a printing mechanism. In case of the electrophotography system, a tilt and curvature of a scanning line by a deflection scanning unit are caused by nonuniformity of a lens and a displacement of a mounting position of the deflection scanning unit required to expose a photosensitive member, a displacement of a mounting position of the deflection scanning unit on an image forming apparatus main body. More specifically, a position of an actual scanning line by the deflection scanning unit displaces from its ideal position, that is, misregistration occurs. Especially, in case of a multi-color image forming apparatus which uses a plurality of photosensitive members, a tilt and curvature (misregistration) of a scanning line may be different for respective colors. As a result, when toner images are transferred onto a transfer member or printing material to be superposed on each other, relative positions of these images are displaced, thus causing color misregistration, that is, image quality deterioration.
As a method for coping against misregistration of a scanning line and color misregistration caused as a result of the misregistration, a technique of Japanese Patent Laid-Open No. 2003-241131 has been proposed. Japanese Patent Laid-Open No. 2003-241131 has proposed the technique for measuring the magnitude of a tilt of a scanning line using an optical sensor in an assembling process of a deflection scanning device in an image forming apparatus main body, and adjusting the tilt of the scanning line by mechanically adjusting the tilt of the deflection scanning device based on the measurement result.
However, since such mechanical adjustment requires a high-precision adjustment device and movable members, cost may increase, and it is difficult to apply this technique to an inexpensive personal image forming apparatus. In a multi-color image forming apparatus, in recent years, in order to attain a cost reduction, a common deflection scanning device is often used to scan the surfaces of a plurality of photosensitive members corresponding to different colors. In this case, it is difficult for the technique described in Japanese Patent Laid-Open No. 2003-241131 to adjust a scanning line for respective colors.
A method of electrically correcting a tilt and curvature of a scanning line in place of such mechanical adjustment (correction) has been proposed. Japanese Patent Laid-Open No. 2004-170755 has proposed a method of measuring the magnitudes of a tilt and curvature of a scanning line using an optical sensor, correcting bitmap image data to cancel them based on the measurement result, and forming an image using the corrected image data. Since this method electrically corrects a scanning line by processing bitmap image data based on the measurement result, the need for mechanical adjustment members and adjustment processes at the time of assembling can be obviated, thus coping with misregistration of the scanning line at lower cost than the method described in Japanese Patent Laid-Open No. 2003-241131. The misregistration correction by Japanese Patent Laid-Open No. 2004-170755 is divided into correction for one pixel unit and that for less than one pixel. In the correction for one pixel unit, positions of respective pixels of image data are offset in a sub-scanning direction by a correction amount for one pixel unit in accordance with correction amounts of a tilt and curvature of a scanning line. In the correction for less than one pixel, a tone value of each pixel of image data and a pixel value of a pixel which neighbors a pixel of interest in the sub-scanning direction are adjusted. With this correction for less than one pixel, an image corrected by the correction for one pixel unit is smoothed.
However, when the correction based on the method of Japanese Patent Laid-Open No. 2004-170755 is applied to image data of a fine pattern image including patterns such as thin lines and dots which are repeated in short cycles (that is, they are regularly repeated and have a high spatial frequency), a fine pattern image to be formed may suffer density unevenness.
FIGS. 24A to 24D show uneven density that occurs in a fine pattern image. In FIGS. 24A to 24D, tone values of respective pixels are expressed by numerical values ranging from 0 to 100 (%). FIG. 24A shows image data corresponding to a part of a fine pattern image including a fine line (line image) of a 2-dot width along the scanning direction. The pattern shown in FIG. 24A is repeated over the fine pattern image. FIG. 24B shows an example of image data obtained when the correction based on the method of Japanese Patent Laid-Open No. 2004-170755 is applied to the image data shown in FIG. 24A. In general, in an electrophotographic image forming apparatus, tone values less than one pixel are formed by pulse width modulation (PWM). When an image is formed on a printing material using the corrected image data shown in FIG. 24B, an image shown in FIG. 24C is formed.
As shown in FIG. 24A, although the width of the line (density of the line image) included in the input image is constant along the scanning direction, the densities of the line image actually formed on the printing material may become uneven in the scanning direction, as shown in FIG. 24C. That is, in the image formed based on the corrected image data, the densities of the line image are changed for respective positions (scanning positions) p0 to p10 in the scanning direction, and become uneven in the scanning direction, as shown in FIG. 24D. This is caused by the nonlinear relationship between the widths of pulses generated by the PWM and laser light amounts in the electrophotographic image forming apparatus. Furthermore, upon forming a dot having a size not more than one dot, such unevenness is caused by the influence of nonlinear factors during processes of exposure-development-transfer-fixing. For these reasons, tone values of respective pixels in the image data, and actually formed dot sizes and densities do not have a linear relationship, thus forming the line with the uneven width.
For example, compared to ideal dots formed at the scanning positions p0 and p10, small dots corresponding to a tone value=10% are destabilized due to electrophotography characteristics and are finally nearly not formed, and densities tend to be lower at the scanning positions p1 and p9. On the other hand, at the scanning positions p3 to p7, as a result of stronger coupling of dots which neighbor in a lengthwise direction (a sub-scanning direction perpendicular to the scanning direction), dots are stably formed, and densities tend to be too much higher.
Such density unevenness of the line is not so conspicuous when an image solely includes the line, and no problem is posed. On the other hand, when an image includes a plurality of lines which are repeated at short intervals (fine line pattern), changes in dot shape which form each line are visualized as changes in density. Since such density changes periodically occur in the image, stripe-like density unevenness becomes conspicuous, resulting in image quality deterioration.
In order to cope with the aforementioned problems, Japanese Patent Laid-Open No. 2007-279429 has proposed a method for eliminating density unevenness which may occur in an image to be formed by adjusting a correction amount of an image position for a unit less than one pixel based on a measurement value obtained by reading a test pattern image using a sensor.
The method of eliminating density unevenness (FIGS. 24A to 24D), which may occur upon forming a line image, by the technique of Japanese Patent Laid-Open No. 2007-279429 will be described below with reference to FIGS. 25A to 25D. As shown in FIG. 24D, compared to the ideal dots at the scanning positions p0 and p10, the dots at the scanning positions p1 and p9 have lower densities, and those at the scanning positions p3 to p7 have higher densities. For this reason, the image data shown in FIG. 24D is adjusted to values shown in FIG. 25B.
In FIG. 25B, in order to increase dot densities at the scanning positions p1 and p9, dots at barycentric positions in the lengthwise direction (sub-scanning direction) are left unchanged, and densities of upper and lower neighboring dots having tone values=90% and 10% are increased at an equal ratio. More specifically, the densities of these dots are increased at a ratio of about 5%. On the other hand, in order to decrease dot densities at the scanning positions p3 to p7, barycentric positions in the lengthwise direction of the line are left unchanged, and densities of dots having a tone value=100% at the barycentric positions are decreased by an amount higher than the ideal dots at the scanning positions p0 and p10. More specifically, the densities of these dots are decreased at a ratio of about 30%.
FIG. 25C shows an image (image to be formed), which is formed based on the image data shown in FIG. 25B. As can be seen from FIG. 25C, dots at the barycentric positions of the like at the scanning position p5 and the like become smaller than those in FIG. 24C. As a result, as can be seen from FIG. 25D, the densities at the scanning positions p0 to p10 are nearly even, and density unevenness in the image to be formed is eliminated.
By correcting the position of the image to be formed using the technique of Japanese Patent Laid-Open No. 2007-279429, the aforementioned density unevenness can be eliminated when a unicolor fine pattern image is formed. However, when an image in a color mixture state (multi-color image) is formed by superposing a plurality of images of different colors, changes in color mixture state caused by the correction of the positions of the image to be formed may cause color misregistration in the image to be formed.
In general, when a dot of a mixed specific color is formed by superposing dots of different colors, a color effect of a formed dot changes depending on a degree of overlapping of dots of different colors. For example, in a color mixture state based on subtractive color mixture in which dots of different colors perfectly overlap each other and that based on juxtaposition color mixture in which dots of different colors are juxtaposed, the subtractive color mixture forms a color having a darker color effect than the juxtaposition color mixture. For this reason, for example, when degrees of overlapping of dots of different colors are even in an input image, if dot shapes are changed for the purpose of position correction of an image to be formed like in Japanese Patent Laid-Open No. 2007-279429, degrees of overlapping of dots among different colors are changed, thus changing the color mixture state. As a result, the color effects of dots in color mixture states are changed to different states for respective dots, and such changes in color effect cause color unevenness in an image to be formed.
Such changes in color effects may be visualized when an input image is a fine pattern image. For example, when a single line is formed by mixing a plurality of colors, changes in color caused by misregistration correction of images to be formed for respective colors are not so conspicuous, and do not pose a problem. On the other hand, in case of a fine pattern image in which such lines repetitively appear at short interval in an image, different changes in dot shape appear as changes in color, and changes in color periodically occur in an image to be formed due to repetition of lines. That is, when an input image is a fine pattern image, stripe-like color unevenness is generated in an image to be formed, resulting in image quality deterioration.
Especially, when an input image is in a state in which fine patterns of different colors perfectly overlap each other (a degree of juxtaposition color mixture (to be described later)=0%) or in a state in which fine patterns of different colors do not overlap at all (a degree of juxtaposition color mixture (to be described later)=100%), such color unevenness readily occurs. In this case, even when respective dot shapes are changed slightly upon misregistration correction of images to be formed for respective colors, color mixture states of different color effects are generated.